Earthquakes

The discovery of sediment layers in a seaside cave represents the longest record of tsunamis in the Indian Ocean, and the clearest record of tsunamis anywhere in the world.

When the December 26, 2004 Indian Ocean earthquake occurred off the west coast of Sumatra, Indonesia, the 9.1 magnitude event — the third-largest tremor ever recorded on a seismograph — was so strong that it caused the entire planet to vibrate by as much as 0.4 inches. The quake triggered a series of devastating tsunamis that killed up to 280,000 people in fourteen countries, inundating some coastal communities with 100-foot-tall waves. The tsunamis are now regarded as being among the deadliest natural disasters in all of recorded history.

Motivated to better understand quake and tsunami dynamics, scientists Charles Rubin, Benjamin Horton, and their colleagues have been studying the seismic history of the region. Archaeologist Patrick Daly at the Earth Observatory of Singapore (EOS) suggested that they excavate a sea cave about 22 miles south of Banda Aceh, Sumatra.

Analysis of the find, published in the journal Nature Communications, provides a 5,000-year-long sedimentary snapshot of tsunamis in the region. The record shows that eleven tsunamis were generated between 7,900 and 2,900 years ago by earthquakes along the Sunda Megathrust, a 3,300-mile-long fault running from Myanmar to Sumatra along the floor of the Indian Ocean.

The investigation additionally determined that there were two tsunami-free millennia during the 5,000 years, and one century in which four tsunamis struck the coast. The scientists could see that smaller tsunamis tend to occur relatively close together, followed by long dormant periods. These, in turn, tend to be followed by very strong quakes and tsunamis, such as the one that struck in 2004.

According to the researchers, the 5,000-year record of tsunamis represents the first such discovery in a sea cave, the first record of tsunamis over this long of a period in the Indian Ocean, and the clearest record of tsunamis from anywhere in the world.

The stratigraphy of the sea cave in Sumatra excavated by scientists from the Earth Observatory of Singapore, Rutgers, and other institutions shows lighter bands of sand deposited by tsunamis over a period of 5,000 years and darker bands of organic material, largely consisting of bat guano.